Transmissions of data over both wired channels and wireless channels are susceptible to error, as every communication channel carries some amount of noise that can corrupt the transmitted signal. Because data transmission has become a foundation for many technologies relied upon by modern society, the problems caused by communication channel noise are significant and increasingly important. Historically, to address errors in a packet transmission, error detecting (ED) codes such as a cyclic redundancy check (CRC) have been used by a signal receiver to detect whether there is an error in the packet. If an error is detected, the receiver can request retransmission of the packet to ensure that the transmission is received in full.
Based on this concept of retransmission, automatic repeat request (ARQ) schemes utilize positive acknowledgment (ACK) and negative acknowledgment (NACK) feedback from the receiver to improve link reliability. Traditionally, data packets are repeatedly transmitted until either the packet is decoded successfully or the predefined maximum number of repetitions is exceeded. A basic ARQ scheme often performs very poorly, however, because retransmissions are susceptible to the same communication channel noise that causes initial transmission errors, so throughput deteriorates as channel noise increases.
Accordingly, hybrid-ARQ (or HARQ) schemes have been developed that combine conventional ARQ with forward error correction (FEC) to further improve performance of the transmission. In Type I hybrid-ARQ, FEC bits, in addition to the ED bits, are transmitted with the information bits, and provide the receiver with the ability to correct some errors without requiring retransmission(s). In Type II hybrid-ARQ schemes, unsuccessful transmissions are used along with the latest packet received to improve decoding and error detection.
Code combining (CC) ARQ is classified as a Type II hybrid-ARQ scheme. Using CC-ARQ, the same information packet is sent during successive re-transmissions, and maximal ratio combining (MRC) is applied at the receiver to combine the received copies before decoding. The modulated symbols used in the successive transmission remain the same as in the first transmission, which is required for the combining operation.
The integration of a modulation process with the ARQ operation was subsequently proposed to utilize the memory of the successive transmissions and to increase Euclidean distances among the codewords by changing the constellation from one transmission to another. In this regard, manipulation of the mapping rules from bits to signal constellation points over re-transmissions can significantly enhance the performance of this hybrid-ARQ scheme. Based on the observation that different protection levels are offered to different bits in a higher order modulation scheme, the constellation rearrangement (CoRe) scheme was proposed. The CoRe scheme averages the reliability of different bits for 16-QAM (Quadrature Amplitude Modulation) over successive transmissions by swapping the position and/or negation of the least significant bits (LSB), as shown in Table I.
TABLE I16-QAM CONSTELLATION REARRANGEMENTTransmission iBit Seq. Pi (m)Operation1i1q1i2q2Gray-encoded mapping2i2q2i1q1Circ. shift of 2 and inv. of i1q13i2q2i1q1Circular shift of 24i1q1i2q2Inversion of i2q25, 6, . . .—Repeat 1-4
CoRe-ARQ has been adopted in the high speed down link packet access (HSDPA) transmission mode of the universal mobile telecommunications systems (UMTS) and in Institute of Electrical and Electronics Engineers (IEEE) 802.16m standard.
Despite the improvements provided by ARQ, hybrid-ARQ, and the more specific CC-ARQ and CoRe-ARQ, the rapidly growing reliance on wireless communication provides a need for further increasing the resilience and throughput of communication channel transmissions.